I'm looking for ideas as far as getting some spring options in the rear. I have FK coilovers 400F/350R right now, and I'm considering using SWIFT coilover springs (at least for the front) with revalved Konis, but I may try to fab some rear perches that would allow me to fit standard diameter coilover springs on the rear control arms.

What have you guys done to get 400+ spring rates at the rear? What are the options without a custom bottom spring seat? Any spring seat I can source somewhere without making one? (My FKs are installed with the adjustable perch at the top, and the spring fits in the original control arm location at the bottom). I thought about bolting a machined spring perch through the arm, sandwiched with a plate (big washer) underneat.

What spring rates would you suggest for mostly autocross, and maybe some road racing? I'm thinking about 500F/500R as a starting point. I have 21mm F and 24mm R sway bars, and will run in FSP next year on C71 Z214 in 225/45/15 if budget allows. A Torsen LSD will be installed. The car was close to neutral this year, but could use a little more stiffness at the rear.

KW coilovers have the spring seat at the bottom like you are talking about.
my old FS thread has some pictures of the orignal seats, plus some custom ones i machined to gain some height.FS Ford Racing coilovers, aka KW V1

Thanks for the pics, from what I can see you use the control arm hole as centering and its top edge as the vertical contact surface. Are the 3 screws for tightening the perch to the arm with the black washer from underneath? (I just want to eliminate any possible clunking if it's not tightened, stock springs clunk if rubber is removed from the bottom coil)

Any input on existing spring diameters and heights commonly available to test different spring rates? I guess I will use my FK perch at top and fit a machined perch for the same I.D. at the bottom. That KW bottom perch is 3 parts right? Male threaded top part with hexagonal hole accessible from underneath, female threaded bottom part and washer to tighten the part from underneath? Am I right on this?

You can get those perches on ebay for under $25 for a pair. I used a 6" x 2.5 x 400 lb rear spring with a Hypercoil Tender spring to keep the spring from falling out. Its slightly lower than the stock SVT springs. I wish it were a tad lower. No rattles and feels good on the street.

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